We encountered this "dilemma" on our last trip to Disney. Our son turned 3 just 3 weeks before our trip. It was tempting to say he was still 2, but DH & I agreed that it would be wrong. I will say, though, that if you've been to Disney with your child, they have his birthdate in their records. When planning that last trip, when I made reservations, the CM asked if DH, DD, & DS (all by name) would be joining me. Then she proceeded to tell me the kids' birthdates. Good thing we'd decided NOT to lie!
The only "dishonest" thing we did was regarding the Fantasmic dinner package at Mama Melrose's. My DS was EXTREMELY picky -- he didn't eat any meat, cheese, pasta, or veggies. He lived off of peanut butter sandwiches or pb crackers and fruit. When I made the ADRs (PSs at the time), I asked if the restaurant could make DS a peanut butter sandwich. I'd read on these boards that most restaurants were very accommodating & could/would make special things not on the menu. He transferred me to the actual restaurant. They said they absolutely could NOT make a pb sandwich. I called back to complete my PS. I asked the CM if I had to pay $9.99 for DS for the Fantasmic dinner package if there was NOTHING he would eat. If they would have made him a sandwich, I would have GLADLY paid $9.99 for it. The CM said, "I'm just going to put him in the computer as being 2." Turned out, though, that DS slept in his stroller through that entire meal. Then, we fed him crackers & fruit after he woke up later in the park. I would not have done that at a buffet or pre-plated meal where we fed him off of our plates. That wouldn't have been fair. But we didn't get anything that we didn't pay for, really.
As Jiminy Crickett says, "Always let your conscience be your guide."